The concept and objectives of high consistency pulping of paper making stock are relatively old in the paper industry. See, for example, the paper entitled "High Consistency Pulping and Pumping" presented by an employee of the assignee of this application which appeared in Paper Mill News for June 21, 1947, and which reported laboratory experiments wherein paper stock was successfully pulped at 16.5 percent consistency. Successful commercial operation at such consistencies, however, has still remained elusive, as is relatively easy to understand when it is recognized that paper making stock at 15 percent consistency has such low fluidity that an average man can walk on the surface of a pool of it without sinking more than an inch or two.
The potential advantages of such an apparatus and method have also been well recognized. For example, since the cost of stock preparation is ultimately computed on the basis of units of oven dry fiber, pulping at high consistencies should be more efficient and less expensive, even if it might take longer for a given gallonage, in terms of both power and steam consumption for pulping at elevated temperatures.
The art has proposed a considerable variety of forms of apparatus for pulping at high consistencies, and one of the dominant theories has been that because of the resistance of such material to flow, successful high consistency pulping apparatus should operate with a kneading, grinding or tearing action. For example, designs have been proposed which appear to have been derived from such other apparatus as barking drums and hammer mills, and in another case from a cement mixer equipped with an internal rotor geared to operate in the opposite direction from the main drum.